Leptospirosis is a common zoonotic disease most prevalent in the tropics and developing world. Death occurs in over 10% of the ~350,000 cases reported worldwide annually. Despite this ubiquity and incidence of disease, much remains unknown about environmental cycling and transmission to humans. Pathogenic Leptospira bacteria contaminate the environment when shed by urine from mammalian hosts into the environment. Humans often contract leptospirosis from contaminated water or from direct exposure to infected urine. However, cases of leptospirosis have been documented from contact with water with little suspected leptospire contamination, suggesting that other potentially important factors affecting Leptospira persistence and transmission need to be identified. Livestock, pets and peridomestic rats as well as weather patterns are thought to play significant roles in environmental cycling and have been correlated to leptospirosis in humans, but direct epidemiological links are missing. The long-term objective of this work is to reduce the number of leptospirosis cases by better understanding links between Leptospira in the environment and leptospirosis in humans. This study will be conducted around Portoviejo City, Ecuador, where leptospirosis cases are common and will involve collection and analyses of environmental, animal and clinical samples. The central hypothesis of this proposal is that specific sources are more important than others for human transmission. Testing this hypothesis will involve addressing two specific aims. The first is to track environmental loads of pathogenic Leptospira over time and search for biotic and abiotic correlates. As leptospirosis is water borne, samples will be collected from the two large rivers near Portoviejo. Physical properties of sampling sites and samples will be obtained and the load of pathogenic Leptospira will be quantified through real- time PCR analysis. Microbial communities in each sample will be characterized by 16S rRNA gene surveys using high-throughput next-generation sequencing technology. The second specific aim will be to link environmental genotypes to clinical genotypes. Clinical samples will be collected, genotyped and compared to genotypes from samples collected from livestock, rats and river samples. These sample sources represent the most significant known sources of leptospirosis infections and thus provide a unique opportunity to link environmental and animal genotypes to human disease.